Recently, as liquid crystal display devices which have high viewing angle characteristics, transverse electric field mode devices (including IPS mode devices and FES mode devices) and vertical alignment (VA) mode devices are used. The VA mode devices are superior to the transverse electric field mode devices in terms of mass production and therefore have been used in a wide variety of TV applications and mobile applications.
The VA mode liquid crystal display devices are generally classified into MVA mode devices (see Patent Document 1) and CPA mode devices (Patent Document 2).
In the MVA mode devices, alignment control means (slit or rib) which has linear portions extending in two directions perpendicular to each other is provided to form four liquid crystal domains between the alignment control means such that the azimuthal angles of the directors which are representative of the respective domains form angles of 45° relative to the polarization axes (transmission axes) of polarizing plates placed in crossed Nicole. Assuming that the azimuthal angle of 0° is identical with the 3 o'clock direction of the clock dial and that the counterclockwise direction is the positive direction, the azimuthal angles of the directors of the four domains are 45°, 135°, 225°, and 315°. This configuration is most preferable in terms of transmittance because linear polarization in the 45° direction relative to the polarization axes is not absorbed by the polarizing plates. Such a configuration in which four domains are formed in one pixel is referred to as “4-domain alignment structure” or simply “4D structure”.
However, the above-described MVA mode is not suitable to small pixels (for example, the shorter side is less than 100 μm, specifically less than 60 μm). For example, when a slit is used as the alignment control means, the width of the slit need to be about 10 μm or more in order to produce a sufficient anchoring force. To form four domains, it is necessary to form in a counter electrode a slit including portions extending in directions which are different from each other by 90° when viewed in a direction normal to the substrate (“<”-shaped slit) and to form two “<”-shaped slits in a pixel electrode such that the two slits are disposed parallel to each other with a certain space therebetween relative to the counter electrode slit that is assumed as the center. Specifically, it is necessary to dispose the sets of three slits each having the width of about 10 μm so as to be in parallel to each other and to extend in the 45°-225° direction and the 135°-315° direction. If this configuration is applied to a pixel with the shorter side being less than 100 μm, the transmittance (luminance) greatly deteriorates because part of the pixel in which the slits (or ribs) are provided does not contribute to the display. In a small-size liquid crystal display device with higher resolution, for example, a 2.4-inch VGA device for use in mobile phones, the pitch of the pixels (row direction×column direction) is, for example, 25.5 μm×76.5 μm. In such small pixels, even formation of the above-described slit is impossible. As a matter of course, if the width of the slit is decreased, a sufficient anchoring force cannot be obtained.
Patent Document 1: Japanese Laid-Open Patent Publication No. H11-242225
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-202511
Patent Document 3: Japanese Laid-Open Patent Publication No. H06-43461
Patent Document 4: Japanese Laid-Open Patent Publication No. 2002-357830
Patent Document 5: Japanese Laid-Open Patent Publication No. 2006-78968